.\" $OpenBSD: SSL_CTX_set_tlsext_use_srtp.3,v 1.3 2018/03/30 19:31:11 schwarze Exp $
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.Dd $Mdocdate: March 30 2018 $
.Dt SSL_CTX_SET_TLSEXT_USE_SRTP 3
.Os
.Sh NAME
.Nm SSL_CTX_set_tlsext_use_srtp ,
.Nm SSL_set_tlsext_use_srtp ,
.Nm SSL_get_srtp_profiles ,
.Nm SSL_get_selected_srtp_profile
.Nd Configure and query SRTP support
.Sh SYNOPSIS
.In openssl/srtp.h
.Ft int
.Fo SSL_CTX_set_tlsext_use_srtp
.Fa "SSL_CTX *ctx"
.Fa "const char *profiles"
.Fc
.Ft int
.Fo SSL_set_tlsext_use_srtp
.Fa "SSL *ssl"
.Fa "const char *profiles"
.Fc
.Ft STACK_OF(SRTP_PROTECTION_PROFILE) *
.Fo SSL_get_srtp_profiles
.Fa "SSL *ssl"
.Fc
.Ft SRTP_PROTECTION_PROFILE *
.Fo SSL_get_selected_srtp_profile
.Fa "SSL *ssl"
.Fc
.Sh DESCRIPTION
SRTP is the Secure Real-Time Transport Protocol.
OpenSSL implements support for the "use_srtp" DTLS extension
defined in RFC5764.
This provides a mechanism for establishing SRTP keying material,
algorithms and parameters using DTLS.
This capability may be used as part of an implementation that
conforms to RFC5763.
OpenSSL does not implement SRTP itself or RFC5763.
Note that OpenSSL does not support the use of SRTP Master Key
Identifiers (MKIs).
Also note that this extension is only supported in DTLS.
Any SRTP configuration is ignored if a TLS connection is attempted.
.Pp
An OpenSSL client wishing to send the "use_srtp" extension should call
.Fn SSL_CTX_set_tlsext_use_srtp
to set its use for all
.Vt SSL
objects subsequently created from
.Fa ctx .
Alternatively a client may call
.Fn SSL_set_tlsext_use_srtp
to set its use for an individual
.Vt SSL
object.
The
.Fa profiles
parameter should point to a NUL-terminated, colon delimited list of
SRTP protection profile names.
.Pp
The currently supported protection profile names are:
.Bl -tag -width Ds
.It Dv SRTP_AES128_CM_SHA1_80
This corresponds to SRTP_AES128_CM_HMAC_SHA1_80 defined in RFC5764.
.It Dv SRTP_AES128_CM_SHA1_32
This corresponds to SRTP_AES128_CM_HMAC_SHA1_32 defined in RFC5764.
.El
.Pp
Supplying an unrecognised protection profile name results in an error.
.Pp
An OpenSSL server wishing to support the "use_srtp" extension should
also call
.Fn SSL_CTX_set_tlsext_use_srtp
or
.Fn SSL_set_tlsext_use_srtp
to indicate the protection profiles that it is willing to negotiate.
.Pp
The currently configured list of protection profiles for either a client
or a server can be obtained by calling
.Fn SSL_get_srtp_profiles .
This returns a stack of
.Vt SRTP_PROTECTION_PROFILE
objects.
The memory pointed to in the return value of this function should not be
freed by the caller.
.Pp
After a handshake has been completed, the negotiated SRTP protection
profile (if any) can be obtained (on the client or the server) by
calling
.Fn SSL_get_selected_srtp_profile .
This function returns
.Dv NULL
if no SRTP protection profile was negotiated.
The memory returned from this function should not be freed by the
caller.
.Pp
If an SRTP protection profile has been successfully negotiated,
then the SRTP keying material (on both the client and server)
should be obtained by calling
.Xr SSL_export_keying_material 3
with a
.Fa label
of
.Qq EXTRACTOR-dtls_srtp ,
a
.Fa context
of
.Dv NULL ,
and a
.Fa use_context
argument of 0.
The total length of keying material obtained should be equal to two
times the sum of the master key length and the salt length as defined
for the protection profile in use.
This provides the client write master key, the server write master key,
the client write master salt and the server write master salt in that
order.
.Sh RETURN VALUES
Contrary to OpenSSL conventions,
.Fn SSL_CTX_set_tlsext_use_srtp
and
.Fn SSL_set_tlsext_use_srtp
return 0 on success or 1 on error.
.Pp
.Fn SSL_get_srtp_profiles
returns a stack of
.Vt SRTP_PROTECTION_PROFILE
objects on success or
.Dv NULL
on error or if no protection profiles have been configured.
.Pp
.Fn SSL_get_selected_srtp_profile
returns a pointer to an
.Vt SRTP_PROTECTION_PROFILE
object if one has been negotiated or
.Dv NULL
otherwise.
.Sh SEE ALSO
.Xr SSL_export_keying_material 3
.Sh HISTORY
These functions first appeared in OpenSSL 1.0.1
and have been available since
.Ox 5.3 .
